The klystron is a well known resonant or narrow band device which utilizes electron beam bunching. In a klystron, which is a velocity-modulation tube, a beam of electrons is emitted from an electron gun, usually comprising a heated electron emitting filament or cathode with an associated focussing electrode. After being accelerated, the electron beam passes through grids in the walls of a reentrant cavity resonator, often called the buncher, in which each electron receives an additional acceleration, either positive or negative, depending upon the phase and magnitude of the gap voltage during the passage of the electron across the gap. The modulated beam, containing electrons of varying velocities, enters a drift space in which the variations in the electron velocity produce density modulation of the electron beam. Because the velocity of each electron is determined by the excitation phase during which it crosses the buncher gap, an electron which was accelerated will overtake an electron which started earlier but was retarded. In this manner, the electrons tend to bunch together as they travel down the drift tube. The bunching effect is maximized at particular drift distances.
In the klystron, the bunched beam passes through the catcher or output cavity resonator, which is located at the point where bunching is at a maximum. Accordingly, the electrons enter the catcher in pulses, one pulse per cycle. In the cathode, the fundamental frequency component of the beam current, as represented by the bunches and intervening regions of low electron density, drives the output resonator into oscillation. With proper adjustment, the amount of signal power required to produce the bunching effect is relatively small, compared with the amount of energy delivered by the electron beam to the catcher. As a result, the klystron tube is usable as a power amplifier. If a portion of the output power from such amplifier is fed back to the input resonator in the correct phase, self-sustained oscillations will be produced.
The power gain in a klystron originates from the combination of velocity modulation and bunching of electrons during their transit time across the drift space.
In the klystron, the electron beam is accelerated by an accelerating voltage between the electron gun and the first grid of the buncher gap. A stream or beam of electrons having a constant velocity and a constant current density is delivered to the first grid and passes into the modulation gap between the first and second buncher grids. In passing across the modulation gap, each electron is either speeded up or slowed down due to the alternating radio frequency voltage between the first and second grids. The phase of the radio frequency field at the time of the electron's transit determines whether the electron is speeded up or slowed down, and to what extent. Thus, the electrons are velocity modulated as the electron beam passes through the second grid and into the drift space.
In the drift space, electrons which were speeded up in the modulation gap begin to catch up with the slower electrons which are ahead of them, thereby resulting in bunching of the electrons. The bunched electron beam passes across the output gap in the output cavity resonator. The bunches form pulses at the modulating frequency. Such pulses deliver power at such frequency to the output cavity resonator. Harmonics are also present in the bunched electron beam, but a klystron normally uses only the fundamental frequency to excite the output cavity resonator.
The klystron is a resonant, single frequency device having a very narrow output band. For certain important purposes, it would be very advantageous to provide a broad-band beam buncher which would have a broad output band, so that the frequency of the modulating signal could be varied over a wide range, to produce output bunches or pulses over a correspondingly wide range.